This section provides background information related to the present disclosure which is not necessarily prior art.
In transformer-based uninterruptible power supply systems (UPS systems) such as UPS system 100 (FIG. 1) having a rectifier input 101 coupled to an input transformer 102 with a Y secondary 104, the Y secondary 104 often has a high resistance ground connection, often referred to as an HRG. That is, as shown in FIG. 1, Y secondary has an HRG 106 with a resistance referred to herein as HRG resistance 108 connected between the common connection 110 of the Y secondary 104 and actual ground. The hot phases Y-1, Y-2, Y-3 of Y secondary 104 are coupled to a rectifier input 101 of UPS system 100 and provide the AC power input to UPS system 100.
When there is no ground fault in the UPS system 100, the 3 phase currents in power lines 112, 114, 116 from the three hot phases Y-1, Y-2, Y-3 of Y secondary 104 will sum to zero. Thus, a current probe 118 coupled around power lines 112, 114, 116 will see zero current.
When there is a ground fault, shown as ground fault 120 in FIG. 1, on one of power lines 112, 114 or 116, the current flowing through the HRG resistance 108 is measured as part of the process of diagnosing the short. In the example shown in FIG. 1, a meter 122 is coupled to current probe 118 to read the current sensed by current probe 118. When current probe 118, which is coupled around power lines 112, 114, 116, is located between Y-secondary 104 and ground fault 120, current probe 118 will see only the HRG current (as the three phase currents sum to zero) which will be read by meter 122 which for example displays the current it reads. When current probe 118 is located between ground fault 120 and UPS system 100, current probe 118 does not see the HRG current and thus the current read by meter 122 from current probe 118 will be zero.
To locate ground fault 120, current probe 118 is positioned at different locations along power lines 112, 114, 116 typically by starting adjacent Y-secondary 104 and moving current probe 118 along power lines 112, 114, 116 until the current seen by current probe 118 goes to zero. This occurs when current probe 118 is moved across power lines 112, 114, 116 where ground fault 120 has occurred. It should be understood that current probe 118 could be moved along power lines 112, 114, 116 in the opposite direction and the location of ground fault 120 then identified when the current seen by current probe 118 goes from zero to the HRG current. In some cases, the resistance value of the HRG resistance 108 can be switched making it easier to locate the fault as the meter readings change as the HRG resistance value changes and the changing meter readings are more noticeable.
The foregoing is also applicable to a transformer-based UPS system having an inverter output coupled to a Y-primary of an output transformer which has an HRG. It should be understood that the UPS system have both an n input transformer an output transformer having a Y-primary, with the input of the rectifier of the UPS system coupled to the Y-secondary of the input transformer and the output of the inverter of the UPS system coupled to the Y-secondary of the output transformer, with both the input transformer and the output transformer having an HRG. The hot phases of the Y-secondary of the input transformer and of the Y-primary of the output transformer will be referred to generically herein as phases φa, φb, φc. The voltage on phase φb lags the voltage on phase φa by 120 degrees and the voltage on phase φc lags the voltage on phase φa a by 240 degrees.
The foregoing discussion assumed that there is only real current flowing in phases φa, φb, φc. In modern UPS systems, this is not the case as the capacitors in the switched circuits of the UPS systems, the EMI capacitors in particular of the EMI filters of the UPS systems, have an appreciable amount of reactive current flowing through them, typically referred to as charging current.
Consequently, current probe 118 will also see this non-zero charging current in the power lines for phases φa, φb, φc which will then be read by meter 122. As a result, while current probe 118 will see a change in the amount of current as it is moved across the location of ground fault 120, this change will not be between zero and the amount of the HRG current. Rather, the change will be between the charging current and the sum of the charging current and the HRG current.